1. Field of the Invention
The present invention relates to a file repair method for Multimedia Broadcast Multicast Service (MBMS). More particularly, the present invention relates to a file repair method for MBMS which is applicable to a Universal Mobile Telecommunications System (UMTS) network.
2. Description of Related Art
The multicasting technique (one-source-many-destination) is widely utilized for multimedia content delivery in the Internet. In the recent years, mobile telecommunication networks have been integrated with the Internet, and the Third Generation Partnership Project (3GPP) proposed the Multimedia Broadcast Multicast Service (MBMS) for multicasting multimedia content over the Universal Mobile Telecommunications System (UMTS), so as to save network resources on the core network and radio access network.
The MBMS system architecture based on the UMTS packet-switched service domain, including a Broadcast Multicast-Service Center (BM-SC) and a plurality of network nodes such as Gateway GPRS Support Nodes (GGSN), Serving GPRS Support Nodes (SGSN), and Radio Network Controllers (RNC). Wherein the BM-SC is connected to one or more GGSN, and the GGSN connected to the BM-SC is the downstream node of the BM-SC. Each GGSN is connected to one or more SGSN, and the SGSN connected to the GGSN is the downstream node of the GGSN. Each SGSN is connected to RNCs of the corresponding routing areas, and the RNCs in the routing areas are considered the downstream nodes of the SGSN. To improve the efficiency of the MBMS content transmission, 3GPP defines an MBMS Bearer Service to maintain the optimal delivery path of data transmission. The MBMS Bearer Service defines two context structures, MBMS Bearer Context and MBMS UE Context, stored in all network nodes involved in data transmission between the BM-SC and user equipments. Wherein MBMS Bearer Context describes the MBMS Bearer Service-specific information, such as multicast addresses, quality of service, and a list of the downstream nodes to which the current network node will transmit the MBMS content; while the MBMS UE Context stores the related information of a user equipment, which is created through the MBMS Multicast Service Activation procedure.
However, when an MBMS file is multicasted by the BM-SC to the user equipments, data loss or data corruption may occur due to bad wireless channel condition, network traffic congestion, or user mobility. Presently, the file repair mechanisms for MBMS specified by 3GPP can be categorized into unicast repair mechanism and multicast repair mechanism. When the user equipment held by a user confirms that the BM-SC has completed transmitting the file, the user equipment checks whether there is data loss or corruption in the MBMS file. If so, after a particular delay time, the user equipment sends a repair request message to the BM-SC through a TCP session. The repair request message is encapsulated by the HyperText Transfer Protocol (HTTP) GET method. The delay time is to prevent that many repair request messages are sent to the BM-SC at the same time. The BM-SC transmits the repair data requesting by the user equipment through unicast repair mechanism or multicast repair mechanism.
According to unicast repair mechanism, once the BM-SC receives a repair request message sent by the user equipment, the BM-SC replies a repair response message with the requested repair data to achieve the purpose of file repair. Referring to FIG. 1, after the user equipment 110 confirms that the BM-SC 120 has completed transmitting the file and finds a repair block 140 is lost by checking the received data file 130, the user equipment 110 sends a repair request message to the BM-SC 120. After receiving the repair request message, the BM-SC 120 sends the repair response message with the repair block 140 requesting by the user equipment 110 back to the user equipment 110, so as to complete the operation of file repair.
According to the multicast repair mechanism, after the BM-SC receives the repair request message, the BM-SC sends the repair response message with the Uniform Resource Identifier (URI) of a session description file (containing the bearer service information for transmitting repair blocks) back to the user equipment to notify the user equipment that the BM-SC will multicast the repair blocks to the user equipment in which MBMS Bearer Service. In the multicast repair mechanism, the BM-SC can determine whether to multicast the repair block to the user equipment in the original MBMS Bearer Service for file downloading or to request the user equipment to join another MBMS multicast group for receiving the repair block.
FIG. 2 is a diagram illustrating the multicast repair mechanism wherein an original MBMS Bearer Service for file downloading is used for sending repair blocks. After the user equipments 210, 220, and 230 have confirmed that the BM-SC 240 has completed transmitting the file, the user equipments 210 and 220 send repair request messages to the BM-SC 240 to notify the BM-SC 240 that the required data are repair block 213 and repair block 223 by checking the received data file 211 and data file 221, respectively. After a delay time, the BM-SC 240 starts to transmit the repair blocks 213 and 223 to all the user equipments through the MBMS Bearer Service established previously. Thus, the user equipment 230 also receives the repair blocks 213 and 223 that have been previously correctly received, and the user equipment 210 receives the unrequested repair block 223, similarly, the user equipment 220 receives the unrequested repair block 213.
FIG. 3 is a diagram illustrating the multicast repair mechanism wherein a new MBMS Bearer Service is used for sending repair blocks. After the user equipments 310, 320, and 330 confirms that the BM-SC 340 has completed transmitting the file. The user equipments 310 and 330 find that there are data loss by checking the MBMS files 311 and 331, respectively, and then send repair request messages to the BM-SC 340 to request the repair blocks 313 and 333. After receiving the repair request messages, the BM-SC 340 notifies the user equipments 310 and 330 in the repair response message that the repair blocks will be transmitted in another MBMS Bearer Service. Then the user equipments 310 and 330 have to use MBMS Multicast Service Activation procedure to join the new MBMS multicast group. After a particular delay time, the BM-SC 340 executes an MBMS Session Start procedure to establish all the bearer resources between the BM-SC 340 and the user equipments 310, 330. After establishing the bearer resources, the BM-SC 340 starts to transmit the repair blocks. In this file repair mechanism, the BM-SC 340 does not send the repair blocks (that have been previously correctly received) to the user equipment 320. However, the user equipments 310 and 330 still receive unrequested repair block.
As described above, while using unicast repair mechanism, the repair data is individually unicasted to the user equipment, and when a plurality of user equipments require the same data, the multiple copies of repair data are transmitted many times in the core network. In the multicast repair mechanism using original MBMS Bearer Service for file downloading, the data required by user equipment is multicasted to all the user equipments through the original MBMS Bearer Service for file downloading. Hence, it imposes unnecessary user equipment processing overhead and introduces unnecessary repair data transmission in the UMTS network. In addition, according to the multicast repair mechanism through new MBMS Bearer Service, all the user equipments that request repair blocks are required to join another MBMS multicast group, and the repair data is transmitted within the new MBMS Bearer Service, so that only those user equipments in the new MBMS group receive the repair data and network resources can be saved greatly. However, the user equipment has to spend a lot of signaling to join the new MBMS Bearer Service in this mechanism. All the three mechanisms described above may introduce extra network traffic and signal overheads in the UMTS network.